Evaporative media systems, for example direct evaporative coolers, are frequently used in commercial and industrial HVAC systems, including applications for data centers and power plant turbine inlet cooling. Evaporative media systems consume less energy than conventional cooling equipment and are increasingly being used to supplement and occasionally replace conventional cooling equipment. In operation, evaporative media systems use the enthalpy of vaporization of water as a means to cool and humidify air. Typically, this is accomplished by flowing air directly through a media wetted with water. As air passes through the wetted media, water evaporates by taking energy from the air to vaporize the water. Accordingly, the air temperature exiting the wetted media is reduced and the humidity is increased while the energy or enthalpy of the exiting air remains the same as the entering air. This type of a process is often referred to as adiabatic cooling.
It is desirable to not cycle a wetted media stage in an evaporative media system more often than absolutely necessary. This is because as the water dries out, any solids in the water will be deposited on the media. This scale build-up is a significant wear-out mechanism for wetted media. Even so, many feedback based control schemes for activating or staging the media stages to meet an output demand tend to cycle the media stages excessively. This circumstance exists because many feedback based control schemes are effectively always performing an “experiment” on the system and adjusting the output accordingly. For example, if the output is too high, the controller would adjust the command signal to lower the output, if it is too low, it would act to increase the output. Since the output of an evaporative media system needs to be discreet (i.e. each media stage is either dry or completely wet) in order to minimize scale build-up, the control system cannot track this analog signal. As a consequence the analog signal will go up until the entire system or a stage of the system is turned on. If this activation causes an overshoot of the setpoint, the analog signal will decrease until it is turned off, and the cycle repeats.
If the system is a single-stage unit, the addition of the some hysteresis to the control loop may be enough to stabilize the output. However, if the unit has more output levels than one, satisfactory control becomes more difficult. For example, a 1/3, 2/3 unit has two stages that can be on or off with one stage (the 2/3) being twice the size of the other (the 1/3). Such a unit has four output states: off, 1/3, 2/3 or 3/3. Improvements in staging control for multiple stage evaporative cooling systems are desired.